Fiber reinforced polymer beams, such as leaf springs, may be produced by various means, including filament winding, pultrusion, and the individual placement by hand or machine of multiple layers of resin impregnated fibers. (See for example U.S. Pat. Nos. 4,374,689; 4,376,669; 4,529,139; and 4,414,049.) While the individual placing of each layer of fibers offers great freedom in shaping the beam, this approach is relatively slow and expensive. The alternatives of filament winding and pultrusion are faster and less expensive, but inherently result in beams of constant cross sectional area unless supplemented by subsequent processes to reshape the beam and trim away extraneous material. These supplemental processes result in fiber discontinuity and the loss of material as unrecoverable scrap. (See Gottenberg, W. G. and K. H. Lo; Glass Fiber Reinforced Epoxy Leaf Spring Design; 38th Annual Conference, Reinforced Plastics/Composites Institute, The Society of the Plastics Industry, Inc., Febr. 7-11, 1983.)
In the pultrusion process, fibers are drawn through a resin bath and then into a heated die for curing. This results in a beam of constant cross sectional area and shape. If the die is not heated, the resin impregnated material will remain pliable, allowing it to be reshaped and cured in a compression mold. The resulting beam may have a varying cross sectional shape throughout its length, but will continue to have a constant cross sectional area unless trimmed.
Filament winding, like pultrusion, involves drawing fibers through a resin bath. Then, typically, these fibers are wound continuously around and around a mandrel which may be oval (or "football") shaped in cross section. This forms, at one time, two springs connected to each other at both ends. These connections are severed to form individual springs. The fibers may be wound into a die to form a beam of varying cross sectional shape but, as with pultruded beams, the continuity of fiber from one end to the other results in constant cross sectional area unless trimmed.
The disadvantage of a constant cross sectional area may be understood be referring to an article by Dr. Carl Maier, "For Spring Action, Which Cantilever Beam is Best", Product Engineering, Febr. 17, 1958, pages 83-87. There it was disclosed that optimum beams are characterized by uniform stress distribution over their length. Two such ideal beams with a rectangular cross section are found: 1) triangular--having constant thickness and a triangular profile in the horizontal plane, and 2) parabolic--having constant width and a parabolic profile in th vertical plane. While the triangular shape is impractical to use in most applications, the parabolic shape stores the maximum amount of energy per unit of volume and thus is the ideal in spring design.
A method of forming a parabolic spring by filament winding is disclosed in U.S. Pat. No. 4,414,049. In this method the fibers are wrapped around a set of innermost pins and thereafter a second set of pins are inserted into the winding fixture and the fibers are wrapped around these pins. Next, a third set of pins are inserted and the fibers are wrapped around these pins. Pins continue to be inserted as each wrap of fibers is completed. In this method, once the pins are inserted into the winding fixture they remain in place until the winding process is completed. Once the winding is completed the pins are removed from the winding fixture so that the fixture may be heated to cure the resin.
A heavy-duty beam may incorporate 100 loops or more of fibers. To achieve a smooth and gradual reduction in the thickness of the beam, each set of pins as shown in U.S. Pat. No. 4,414,049 should receive only one loop of fibers. This method would then require 50 sets of pins to create a beam having 100 loops.
The winding fixture of the present invention uses only two sets of translatable pins. As the pins are translatable the pins may be located in a vast number of different configurations within a single fixture. The present invention also allows the two pins of each set of pins to be placed at variable spacings from adjacent one another to widely apart. The present winding fixture also eliminates the need for trimming and the resultant fiber discontinuity and material waste. This method of winding also offers the advantage of having the fiber direction co-directional with the beam surface, rather than with the beam centerline, thereby providing strength in the direction in which stress is induced by bending.